The invention relates to photodetectors and particularly to the field of avalanche photodetectors.
Photodetectors with high quantum efficiencies in the 1.0-1.6 .mu.m wavelength region are expected to find wide use in low-loss wide-bandwidth optical fiber transmission systems as well as in other applications. Avalanche photodetector devices are of interest here because compared with simple junction photodiodes, they allow a considerable increase in the sensitivity of optical receivers. The photon-excited carriers in avalanche devices gain sufficient energy to release new electron-hole pairs by ionization and these new carriers provide gain for the photocurrent. However the noise factor, a measure of the degradation of a photodetector as compared to an ideal noiseless amplifier, increases considerably with the average gain. In an avalanche photodiode the noise factor of the carrier multiplication process depends both on the ratio between the ionization coefficients, i.e. the ionization probability per unit length, for electrons and for holes and on the way the carrier multiplication is initiated. A large difference between ionization coefficients is beneficial for low noise, provided the avalanche is initiated by the carrier type, electron or hole, having the higher ionization coefficient. Ideally, the least noise is obtained for a given gain if the smaller ionization coefficient is zero.
Although silicon exhibits a very large difference between the ionization coefficients of electrons and holes, especially at low fields, the response of silicon devices to photons does not extend much beyond 1.1 microns, being basically limited by the 1.12 eV bandgap energy of the silicon.
Germanium avalanche photodiodes appear to be well suited for detection of photons in the wavelength range of 1.1-1.5 microns. However, germanium has almost equal electron and hole ionization coefficients, causing these devices to suffer from the excess noise of a less-than-ideal carrier multiplication process.
Avalanche photodiodes fabricated out of III-V semiconductor components, having radiation wavelength sensitivities which are adjustable in the region of practical interest, i.e. the low-loss spectral transmission window for optical fibers, also suffer from high noise due to the near equality of ionization coefficients for holes and electrons.
The situation with respect to the state of the art for producing avalanche photodetectors is best summarized by quoting from the article, "Detectors for Light Wave Communication," by H. Melchior in Physics Today, November 1977, pp. 32-39. On page 38, Mr. Melchior states "An ideal detector material would be one in which only one type of carrier--either the holes or the electrons--undergoes ionizing collisions. Finding such a material is a tedious job; because of the lack of technological guidance each material with a suitable bandgap has to be investigated experimentally."